1. Field of the Invention
The present invention relates to a control apparatus for a hydraulically operated vehicular transmission having a hydraulic engaging element for a low-speed transmission train which establishes the low-speed transmission train in cooperation with a one-way clutch, and hydraulic engaging elements for high-speed transmission trains.
2. Description of Related Art
In this kind of transmission, there has hitherto been known the following arrangement as can be seen in Japanese Published Unexamined Patent Application No. 180756/1988. Namely, in the running range, hydraulic oil is always supplied to a hydraulic engaging element for a low-speed transmission train and, also, hydraulic oil supply to, and discharge from, hydraulic engaging elements for high-speed transmission trains are controlled by shift valves. When a hydraulic engaging element for one of the high-speed transmission trains has been engaged through the hydraulic oil supply thereto, the power transmission via the low-speed transmission train is automatically stopped through the operation of the one-way clutch, thereby establishing the high-speed transmission train. When the hydraulic engaging element for the high-speed transmission train has been released through the hydraulic oil discharge therefrom, the low-speed transmission train is established.
Further, in this apparatus, there is provided an accumulator for each of the hydraulic engaging elements. Speed change shocks, i.e., shocks at the time of supply and discharge of the hydraulic oil to and from each of the hydraulic engaging elements are thus arranged to be decreased or alleviated by buffering sudden changes in the pressure by means of each accumulator.
These accumulators are relatively large and, if they are provided for each of the hydraulic engaging elements, the hydraulic control apparatus becomes large and consequently the transmission itself also becomes large. Therefore, it is desired to reduce the physical size of the accumulators or to reduce the number of the accumulators to be used. However, according to an arrangement for this desire, the following problems may occur.
Namely, at the time of speed changing to the low-speed transmission train having the one-way clutch, in which the speed changing is effected only by the disengagement of the hydraulic engaging element for the high-speed transmission train, if there is no accumulator for the hydraulic engaging element in question, the hydraulic engaging element in question will instantly be released by the changing over of a shift valve, and the one-way clutch will be engaged at the same time. As a result, a sudden speed changing is effected with consequent shocks.
In view of the above-described points, the present invention has an object of providing a control apparatus in which the shocks at the time of speed changing to a low-speed transmission train can be alleviated even if there is abolished, or reduced in physical size, an accumulator for buffering the sudden drop in pressure at the time of oil discharge from the hydraulic engaging element for the high-speed transmission train.